(705d) Co-Gasification of Biomass and Fossil Fuel: Synergistic Or Inhibition Effects?

The higher energy density of solid fossil fuels (~25-40 GJ/m³) compared with biomass (~2-10 GJ/m³), coupled with reduced greenhouse gas emission of biomass compared to fossil fuels, make co-feed gasification a feasible bridge between energy production via fossil and renewable resources. Although a significant amount of work has been done on converting solid hydrocarbons to product gases through pyrolysis and gasification, disagreement remains in the literature on how biomass particles and their gas products interact with fossil fuel hydrocarbons during thermochemical conversion.

Lab scale co-pyrolysis and co-gasification of switchgrass and sub-bituminous coal were investigated. Switchgrass ash was rich in potassium (~17wt.% in ash), whereas the coal contained 30.5%wt. dry basis ash rich in silicon (~27%wt. in ash).  Thermogravimetric analysis of two-stage co-gasification of switchgrass and coal with different weight proportions (i.e. coal:biomass= 100:0, 75:25, 50:50, 25:75, and 0:100 %wt.) was first performed at 750, 800, and 900°C (25°C/min heating rate) and atmospheric pressure using N₂ for pyrolysis and CO₂ for gasification (both at 500 ml/min volumetric flow rate). Three stages - (i) drying, (ii) rate controlled by intrinsic kinetics, (iii) rate controlled by intraparticle diffusion - were distinguished during pyrolysis. The average experimental rate of reaction was same than expected if the components were to act independently. Therefore, it is concluded that the switchgrass ash had no synergistic effect on coal pyrolysis.

CO₂ co-gasification results showed that the presence of biomass could either inhibit or enhance coal conversion. Mixtures of 75:25 and 50:50 wt.% coal/biomass reached then final conversions much slower than for pure coal gasification. However, increasing the potassium concentration of the blended sample, due to catalytic effect of potassium on coal carbon active sites, give much faster conversion than expected. The inhibition at low potassium concentrations was due to potassium deactivation caused by secondary reactions with coal minerals. The potassium reacts with coal minerals, like illite and kaolinite, to form a new mineral phase, e.g. kaliophilite (KAlSiO₄). At higher gasification temperature (900°C), the gasification rates were mainly strong functions of temperature, and catalytic synergistic effects or inhibitory reactions between potassium and coal minerals were less important than for gasification at lower temperatures.

Based on thermogravimetric results, a thermal coal with lower ash content (13.5wt.%) and an alternative switchgrass  were chosen to perform single fuel and co-feed steam gasification in a 100mm ID and 1.2m long pilot-scale atmospheric pressure bubbling fluidized bed reactor at an average temperature of 850°C using a steam-to-fuel mass ratio of 2.3 with silica sand as the bed material. The tar content of the process decreased from 9.6 g/m³ for pure switchgrass to 6.6 g/m³ for pure coal and 5.8 g/m³ for a 50:50 wt% coal/biomass mixture. Alkali and alkaline earth metals of biomass ash caused tar cracking/reduction of product gas in the co-gasification experiment. On the other hand, large agglomerated particles were collected in the downstream heat exchanger tube.